Tyre testing method and apparatus

ABSTRACT

A method for testing tires includes setting a tire, with a predefined inflation pressure and load condition, in rotation on a surface with a predefined speed, and applying a pressure against a sidewall of the tire in proximity to a contact area of the tire with the surface. The pressure is applied using a shaped body for deforming the tire sidewall according to a profile including at least two portions with opposite curvatures. A related apparatus includes a surface, an arm for supporting a tire, a device for setting the tire in rotation on the surface, and a shaped body movable transversely with respect to the tire between a non-operative position and an operative position. In the operative position, the shaped body pushes against a sidewall of the tire to deform the tire sidewall a predefined depth with respect to an undeformed profile of the tire sidewall.

[0001] The invention relates to a method for testing tires and to anapparatus for implementation thereof, intended in particular but notexclusively for tires of heavy road haulage vehicles (lorries and thelike).

[0002] A phenomenon typically affecting this kind of tires consists inthe break which reveals itself as a clean cut on the side of the tire,extending circumferentially thereto and along which cutting off of theradial cords forming the carcass of said tire occurs. Said phenomenon isknown to persons skilled in the art by the name of “zipper break”.

[0003] The reinforcing cords of tire carcasses for heavy road haulagegenerally consist of metallic wires and are much stronger than textilecords commonly used in the carcasses of car tires: as a result it istherefore difficult to understand how this type of break occurs, allthat more taking into account that it may happen even only after a smallnumber of kilometres travelled by the tire.

[0004] In other words, said “zipper break” cannot be attributed to thenormal wear to which a tire is subject following prolonged use over timeor in particularly severe conditions (heavy load, rough road surface,etc.), and does not fall within the other known forms of tire breaks.

[0005] Consequently, hitherto it has not been possible to understand andreproduce with the usual methods (computer analysis, experiments, etc.)the conditions which in real life lead to the “zipper break” of tires;this has also rendered impossible to provide tire structures for lorriesand the like, specifically designed to prevent said particular type ofbreak.

[0006] The present invention aims at remedying this situation.

[0007] The invention arises from Applicant's perception that the “zipperbreak” is caused by abnormal and occasional working conditions of thesidewall of the tire, capable of causing a particular fatigue stress inthe carcass structure.

[0008] An example of these stresses is the so-called “kissing”, i.e. theinterference which occurs between two tires mounted side by side on thesame axle (such as the twinned tires in the lorries) in the event of anexcessive load acting on them, or when one of the tires is operatingwith an inflation pressure lower than that of the other tire or both thetires have a pressure which is lower than the nominal operatingpressure.

[0009] Therefore, on the basis of said perception, the present inventionprovides a method for testing tires and an apparatus for carrying outsaid method, suitable for reproducing the conditions which, during use,give rise to the phenomenon of “zipper break”.

[0010] The characterising features of said method and of theabovementioned apparatus are set out in the following claims.

[0011] Said features, together with the effects arising therefrom, willemerge more clearly from the description herebelow of a preferred andnon-exclusive embodiment of the invention, illustrated in theaccompanying drawings, wherein:

[0012]FIG. 1 shows a partial radial cross-section of the interferencezone of a pair of twinned tires in “kissing” condition;

[0013]FIG. 2 shows a top plan view of the interference zone of FIG. 1;

[0014]FIG. 3 shows a perspective view of an apparatus for carrying outthe method according to the invention;

[0015]FIG. 4 shows a cross-sectional view of the apparatus of FIG. 3;

[0016]FIGS. 5 and 6 show in detail two steps of the method according tothe invention;

[0017]FIG. 7 is a diagram showing the behaviour of two different tiresduring the testing method according to the invention.

[0018] With reference to FIGS. 3 to 6, there will be first described anapparatus for carrying out the abovementioned method, indicated in itsentirety by reference sign 1 in FIG. 3.

[0019] Said apparatus is derived from those apparata normally used forrolling tests, in which a tire 2, mounted on a rim 3, is set in rotationon a rotating drum 4 (also called “road-wheel”) thereby simulating theoperation of a tire on the road

[0020] The tire 2, mounted idle on its axle, is set in rotation by thedrum 4, operated by a motor (electrical or of another type) not shown inthe drawings, and the rolling conditions of the tire 2 on the drum 4 maybe varied during the test by modifying the speed of rotation, the loadapplied to the tire, the slip and camber angles given to the tire axis.

[0021] For this purpose, the rim 3 is mounted on an arm 5 which projectsfrom the frame 6 of the apparatus 1, said arm being movable upwards anddownwards along said frame by means of an operating screw 7.

[0022] By varying the height position of said arm 5, a smaller orgreater contact force between the tire 2 and the drum 4 is obtained,thus simulating various loading conditions acting on the wheel of avehicle.

[0023] The Figures do not show the devices for varying the camber andslip angles since they are known and not relevant for the purposes ofunderstanding the invention.

[0024] A shaped disc 10 is arranged in a position underneath the arm 5(as will be explained more clearly below), being mounted idle on avertical support pin 11 such that said disc 10 is free to rotate withrespect to the vertical axis L of such pin 11, which is perpendicular tothe axis of rotation of the tire 2.

[0025] The pin 11 is in turn movable to translate along a horizontalguide 13, by means of a screw/lead nut thread coupling 14 or similardevice; said movement allows the disc 10 to move towards or away fromthe tire 2 in accordance with pre-selected manners for performing thetest.

[0026] Finally, the guide 13 is also vertically displaceable by means ofa set of four screwed columns 16, so as to vary the position of the disc10 along the sidewall of the tire 2.

[0027] According to the method of the invention, the disc 10 isgradually moved towards the tire 2 while the latter is rolling on thedrum 4, starting from an initial position wherein the disc 10 is at adistance from the tire 2 (see FIG. 5).

[0028] When the disc 10 comes into contact with the sidewall of therotating tire, it is the frictional force which sets the disc 10 inrotation.

[0029] At this point of the test, the disc 10 is pushed further againstthe sidewall of the tire, which is thus deformed by it as shown in FIG.6.

[0030] The penetration depth of the disc 10 into the sidewall of thetire 2, following initial contact between them, is a parameter—measuredin mm or in other units of length—which provides an indication of thepenetration degree of the sidewall of a first tire into the sidewall ofa second tire alongside the first one, namely an indication of thedeformation following the abovementioned “kissing” between two adjacenttires.

[0031] In order to better understand this concept, reference should bemade to FIG. 2 which is a plan view showing the deformations of twotwinned tires 21, 22, also shown radially sectioned in FIG. 1; inparticular, the pressure P₁ in the first tire 21 is greater than thepressure P₂ in the second tire 22.

[0032] This is the typical situation that occurs when one of the twotwinned tires has a pressure which is lower than that of the other tire,and therefore the distribution of the load over them is not uniform;this causes an excessive compression (in the cross-section) of the tireswith respect to their normal operating condition, wherein the two tiresare not in contact with each other, such that the sidewall of the tirewith a higher pressure compresses the sidewall of the tire with a lowerpressure, thereby giving rise to the phenomenon of “kissing”.

[0033] As shown in FIG. 2, this situation affects the part of the tires21, 22 which is located in connection with the footprint areas 23, 24 ofthe respective tread bands 25, 26; said areas are indicated with thehatch in FIG. 2 and represent the tire zone where contact with theground occurs.

[0034] In accordance with a preferred embodiment of the invention, thedeformed configuration of the tire 21, whose sidewall urges against thesidewall of the tire 22, may be advantageously reproduced by a circulardisc which has a diameter 27 similar to that of the curvature of thesidewall of the higher-pressure tire.

[0035] The shape of said disc is indicated by the broken circular linein FIG. 2 which corresponds to the shaped disc 10 used in the apparatus1 described above. The height and the profile of the disc 10 aretherefore shaped so as to reproduce the penetration of the sidewall of afirst tire into the sidewall of a second tire arranged alongsidethereto.

[0036] Therefore, by varying the position of the disc 10 with respect tothe sidewall of the tire to be tested, with the method according to theinvention it is possible to reproduce the various “kissing” situationswhich occur in real conditions when two twinned tires are used.

[0037] More specifically, with reference to FIG. 6 it can be seen thatwhen the disc 10 urges against the sidewall of tire 2, the latter isdeformed assuming a profile with curvatures in opposite senses, i.e.concave in certain points and convex in others; this situation isrepresented by the circumferences C₁, C₂ and C₃ (the latter shown onlypartially) in FIG. 6, arranged along the sidewall of the tire 2 anddefining different radii of curvature assumed by said sidewall followingto “kissing”.

[0038] If the disc 10 is pushed further (by means of the lead nut thread14) against the sidewall of the tire, the radii of the threecircumferences C₁, C₂ and C₃ change with respect to those shown in FIG.6; the same thing happens if the disc 10 is moved vertically (by meansof the screw columns 16) with respect to the tire 2, without changingthe distance between the disc 10 rotation axis and the median(equatorial) plane of tire 2.

[0039] By comparing FIGS. 1 and 6, it is possible to appreciate thecorrespondence between the tire sidewall deformation obtained with themethod according to the invention, and that caused by the interferencebetween two tires in “kissing” condition.

[0040] At this point, however, it has to be pointed out that theabovementioned deformation occurs only in the sidewall zone affected by“kissing”: in the remaining part the sidewall has a different, generallyconvex, profile which may be approximated with a circumference such asthat indicated by C₄ in FIG. 5, relating to the undeformed condition ofthe tire.

[0041] Consequently, for each revolution of the tire the sidewallsubjected to the “kissing” phenomenon undergoes a cyclical deformationwhen it passes through the interference zone, resulting in a change ofits configuration from a generally convex one with a substantiallyuniform radius of curvature (such as that of the circumference C₄), intoa configuration wherein there is a concave zone (that of thecircumference C₃) alternate with convex zones having small radii ofcurvature (defined by C₁ and C₂).

[0042] This means that the sidewall of the tire and the components whichform it, i.e. the lining rubber and the carcass wires, are subjected toan alternate stress whose magnitude depends on the degree of “kissing”deformation (caused by an adjacent tire or by the testing disc 10) andhaving a frequency equal to the number of revolutions of the tire in agiven unity of time.

[0043] The Applicants have found that said alternate stress produces afatigue stressing in the sidewall of the tire, and in particular in themetallic carcass cords, thereby leading to the “zipper break” of thesidewall, as described above.

[0044] It should further be observed that said deformation takes placeon the tire sidewall at least along two different planes: asubstantially radial plane (containing the tire rotation axis—FIG. 6)and another one substantially perpendicular to the equatorial plane ofsaid tire (FIG. 2).

[0045] According to Applicant's opinion, without any limitation arisingtherefrom, the break of the cords should be ascribed to a combinedfatigue action, due to the combined compression (brickling) and flexionstresses which are particularly high when the tire is running with aninflation pressure lower than the nominal one, or in particular rollingconditions (for instance, along a curvilinear path).

[0046] The identity between the types of breaks occurred in real lifeand those caused during laboratory tests, was verified by performing acomparative analysis of the fracture sections of the carcass cordsbelonging to tires mounted on lorries and removed from the vehicle after“zipper break”, and the fracture sections of cords of tires tested onthe apparatus according to the invention.

[0047] This analysis has shown that in both cases (testing method andreal conditions) yelding of the cord wires in the tire broken zone isdue to fatigue. As persons skilled in the art know well, in this casethe break of the wires may occur along a clean plane (with angles, forexample, between 90° and 45°) or may have a “flute tip” form.

[0048] These results have been confirmed (and have demonstrated theirreproducibility) in a series of tests which were carried out using theapparatus 1 described above, said tests being explained herein belowwith the aid of some tables.

[0049] Said tests consisted in subjecting tires of different rubbercompound compositions and/or carcass structures to an operating cycle onthe apparatus 1, varying with pre-established steps the penetrationdepth of disc 10 into the sidewall of the tire, the pressure of the tireand the load acting on it.

[0050] In particular, the penetration depth of disc 10 was consideredits position with respect to a reference condition in which said disc istangential to the tire sidewall. This parameter was measured inmillimetres.

[0051] In the tables below, the penetration depth of the disc, referredto as “kissing”, has been assumed as the parameter representing thesimulated degree of interference between two tires in real conditions.

[0052] The first experiment was carried out on 315/80 R22.5 Applicants'tires, generically defined as “model A”.

[0053] In particular, a first tire (I) with gradually increasing loadand pressure levels, following a step by step sequence including, forthe tire size above, a load variation from 3000 to 9000 kg (nominal loadof 4000 kg) and an inflation pressure variation from 5 to 9 bar (nominalpressure of 8 bar), was tested. The results obtained are shown in Table1, from which it can be underlined that there was no “zipper break” ofthe tire, but only a deterioration thereof as a result of sidewallrubber removal from the carcass along the sidewall against which thedisc had acted, after 400 hours. Therefore, the “zipper break” does notdepend on the load variation acting on the tire.

[0054] Thereafter a second tire (II) was tested, starting directly withthe more severe conditions reached by the first tire (I). Also in thiscase there was no “zipper break”, but only deterioration with removal ofthe rubber after 148 hours.

[0055] In other words, the method according to the invention hasdemonstrated with the results of said first test that “zipper break”does not depend on the heaviness of the operating conditions to whichtires are subjected. TABLE 1 315/80 R22.5 tyre—“Model A” Tyre LoadPressure Kissing Speed Duration Type of No. (kg) (bar) (mm) (km/h)(hours) break I 3000 5 15 40 240 I 6000 9 15 30 100 I 9000 9 20 30 60Separation of rubber II 9000 9 20 30 148 Separation of rubber

[0056] Thereafter less severe conditions for the tires were chosen asregards the load, but using inflation testing pressures substantiallylower than the nominal one; the results for the same type of tire(“model A”) used in the first experiment are shown in Table 2. TABLE 2315/80 R22.5 tyre—“Model A” Tyre Load Pressure Kissing Speed DurationType of No. (kg) (bar) (mm) (km/h) (hours) break I 2000 2.5 15 60 24“Zipper break” II 2000 2.5 15 60 7 “Zipper break” III 2000 2.5 15 60 10“Zipper break”

[0057] As can be seen, in these operating conditions after a fairlylimited number of hours of operation (24 h, 7 h and 10 h, respectivelyfor tires I, II, III), “zipper break” of the tires occurred; it shouldbe noted that this happened with a low inflation pressure in accordancewith what was explained before about the fact that, in real conditions,this particular break takes place when one of the two twinned tires hasan inflation pressure lower than the other.

[0058] The behaviour of the tires with regard to this phenomenon (i e.,a greater or smaller duration) generally depends on their structure andtherefore on the carcass type as well as on the rubber compound used forthe manufacture of the various parts of the tires and on the treadpattern.

[0059] This may be easily appreciated from Table 3 which refers to atest cycle carried out by using the same parameters as in Table 2, onApplicants' tires of the same size but with a different tread pattern(“Model B”).

[0060] As can be seen, the number of hours following which “zipperbreak” occurred is substantially greater in this second case. TABLE 3315/80 R22.5 tyre—“Model B” Tyre Load Pressure Kissing Speed DurationType of No. (kg) (bar) (mm) (km/h) (hours) break I 2000 2.5 15 60 116“Zipper break” II 2000 2.5 15 60 85 “Zipper break” III 2000 2.5 15 60 94“Zipper break”

[0061] Finally, as mentioned further above, at the basis of the presentinvention is Applicants' perception that “zipper break” depends on afatigue phenomenon, following the cyclical deformation of the tiresidewall when it is subjected to “kissing”.

[0062] For this purpose special tests were thus carried out duringwhich, all other conditions (i.e., pressure, speed, load) remainingunchanged, there was varied the penetration depth of the test disc 10into the tire sidewall, i.e. the degree of deformation thereof.

[0063] In particular, during the experimentation, it was decided toincrease gradually stepwise the “kissing” value applied to the testtire, after a predefined test period (24 hours).

[0064] The results relating to tests carried out on three 315/80 R22.5tires, “Model C” are shown in Table 4. Said tests clearly show theexistence of a “kissing” level below which zipper break does not occur.

[0065] Indeed, as shown, in the case of two tires (I and II) subjectedto “kissing” values of 30 and 32.5 mm, respectively after 96 and 168(i.e. 4 and 7 days) continuous hours of testing, no fault occurredwhereas after just two hours of testing (again for the same tire I andII) at a higher “kissing” value (40 mm) the “zipper break” occurred.

[0066] Likewise, another tire (III) subjected as from the first step toa “kissing” value of 40 mm, revealed the “zipper break” after only threehours. TABLE 4 315/80 R22.5 tyre—“Model C” Tyre No. Kissing (mm)Duration (hours) Result I 30 96 no fault I 40 2 “Zipper break” II 32.5168 no fault II 40 2 “Zipper break” III 40 3 “Zipper break”

[0067] On the basis of the tests carried out by the Applicants, for eachtested tire it was possible to obtain a curve whose typical trend isshown in the graph of FIG. 7.

[0068] This graph carries along the abscissa axis the duration of thetest carried out on a tire and along the ordinate axis the penetrationdepth (“kissing”) of the disc 10 into the tire sidewall.

[0069] The curves X and Y of FIG. 7 relate respectively to a givenmanufacturing type of a tire, in predetermined load and pressureconditions. More specifically, curve X relates to a Model A tire withnominal load of 9000 kg and operating pressure of 3 bar, while curve Yrelates to a Model B tire.

[0070] As shown, the typical trend of said curves is generallydecreasing, with a profile roughly parabolic characterised by thepresence of a horizontal asymptote (broken line): this means that in thecase of “kissing” values lower than said asymptote, no “zipper break”occurs.

[0071] Furthermore, the graph of FIG. 7 shows the displacement of saidtypical curves following to the different structures of the tires.

[0072] More in detail, in FIG. 7 the duration/penetration depth curvesof two different Applicants' tires X and Y are compared.

[0073] Furthermore, a person skilled in the art may appreciate how thetrend of said curves is similar to that of the well-known Wholer curves,relating to the fatigue of materials subjected to alternate stresses.

[0074] For this purpose it is sufficient to consider that, for example,in the case of tires having the sizes referred to above, a test lastingfor 100 hours at a speed of 60 km/h corresponds to about 1.8×10⁶ cyclesof alternate stress applied to the carcass cords.

[0075] Moreover, the degree of “kissing”, on which the degree ofdeformation of the carcass cords depends, produces in said cords acorresponding state of tension: consequently, along the ordinate axis ofthe graph in FIG. 7 it would be possible to replace the “kissing”parameter (in mm) with the tension (in Pa or kg/mm²), thereby obtaining,also from this point of view, a perfect correspondance with the Wholercurves.

[0076] In other words, the “kissing” deformation test carried out byusing the method according to the invention is equivalent to a test inwhich the tire sidewall and the carcass cords are cyclically stressed bychanging the curvature thereof in accordance with what explained abovein connection with FIGS. 5 and 6.

[0077] Therefore, it follows that a state of fatigue is induced in thesecomponents (sidewall and associated cords) which causes their suddenyielding (collapse) within brief periods and under not particularlysevere conditions (tire load and inflation pressure).

[0078] In the light of the foregoing it has therefore been shown thatthe method according to the invention allows to achieve resultscorresponding to those under real conditions, where the “zipper break”occurs also on tires which are almost new and, more generally, at anymoment during the life of the tire: for this purpose it is sufficientthat the tire operates under “critical” conditions (i.e., above theasymptote of the curves shown in FIG. 7) even for a limited number ofhours.

[0079] Applicants have also found that a “memory effect” exists in thetested tires.

[0080] Indeed, the experiments carried out have shown that it is notnecessary for the tire to be kept continuously under the abovementionedcritical conditions to achieve the “zipper break” thereof; said breakoccurs also if the tire is kept under said conditions for severalperiods of time alternate, at intervals, with normal operatingconditions (i.e., with no “kissing”), provided that the overall sum ofthe partial periods is such as to reach the fatigue limit defined in thediagram according to FIG. 7.

[0081] In this connection, finally, it is important to underline animportant advantage achieved by the apparatus according to theinvention.

[0082] Indeed, as mentioned above, the invention arises from Applicants'perception that the “zipper break” of tires mounted on lorries is causedby the interference which occurs between two twinned tires, as shown inFIG. 1.

[0083] Consequently, on the basis of this perception it would also bepossible to carry out the method of the invention using an apparatus inwhich there are two tires arranged side by side, one of said tirescausing the deformation of the other one.

[0084] Such an apparatus would, however, be much more complex than thatshown in FIG. 3 and therefore also the tests would be more difficult tobe performed since, in order to produce an interference of the desireddegree, it would be necessary to adjust several parameters instead ofthe abovementioned penetration (“kissing”) of the test disc 10.

[0085] Of course, other variations of the invention with respect to thatdescribed hitherto will also be possible.

[0086] In one embodiment, not shown, a load cell is positioned on theaxis of the disc 10 so as to measure the lateral force exerted by saiddisc on the sidewall of the tire to be tested.

[0087] Furthermore, in the example described above the disc 10 ismounted in an idle manner and is driven rotatably by means of thecontact with the tire to be tested. In accordance with a furtherembodiment of the invention said disc, being driven by a motor, is setin rotation at a required speed for carrying out tests where the effectof relative fictional contact thereof with the tire is assessed.

[0088] Moreover, the rounded profile of the external edge of the disccan be different from that shown in the drawings.

[0089] For instance, it can have discontinuities, ribs and/orprojections so as to simulate the presence of a stone or the likesquashed between two adjacent tires, as sometimes occurs under realconditions.

[0090] The disc 10 considered above is movable horizontally, in order tovary the penetration depth into the tire sidewall, and vertically, inorder to deform the sidewall at different heights when required.However, it is possible to provide apparata where the inclination of therotation axis of the disc is also adjustable with respect to themeridian plane of the tire, so as to obtain further experimentaleffects.

[0091] Last, the rolling surface of the tire to be tested, which in theabove apparatus consists of the usual road-wheel, could also be made byusing another equivalent system; for example, use could be made of aconveyor belt (suitably supported in the footprint area of the tire)which would allow tests to be carried out on tires rotating on a flatsurface.

[0092] Finally it is necessary to take into consideration the fact that,although the invention is of particular interest for tires intended forapplications on industrial vehicles, where “zipper breaks” caused bytwinned tires occur, it may nevertheless be applicable more generallyalso to any tire in order to study their behaviour in response tostresses cyclically acting thereon.

1. Testing method for causing a controlled break of a tire sidewall,characterized in that it comprises the steps of: setting in rotation atire (2) on a surface (4) with predefined speed, inflation pressure andload conditions; applying a pressure against said tire sidewall in theproximity of its contact area with said surface (4), by means of ashaped body (10) for deforming said sidewall according to a profilecomprising at least two portions with opposite curvatures (C₁, C₂, C₃),concave and convex.
 2. Method according to claim 1, wherein said shapedbody (10) is moved transversely with respect to the tire (2) between anon-operative position, in which said shaped body (10) is not in contactwith the sidewall of said tire, and an operative position in which saidshaped body (10) is made to advance and penetrate into said sidewall fora predefined depth with respect to the undeformed profile of saidsidewall.
 3. Method according to claim 2, wherein said shaped body (10)is movable along said sidewall of the tire (2) in a radial directionwith respect thereto, so as to deform said sidewall at differentheights.
 4. Method according to claim 3, wherein said shaped body (10)is set in rotation at a predefined speed.
 5. Apparatus for causing acontrolled break in the sidewall of a tire, comprising a surface (4), anarm (5) for supporting said tire (2), means for setting in rotation saidtire on said surface (4), a shaped body (10) movable transversely withrespect to said tire, between a first, non-operative, position and asecond, operative, position in which said shaped body (10) penetratesinto said tire sidewall for a predetermined depth.
 6. Apparatusaccording to claim 5, wherein said shaped body (10) is a rotatable disc.7. Apparatus according to claim 6, wherein said shaped body (10) ismovable along the sidewall of said tire (2) in a radial direction withrespect thereto, so as to deform said sidewall at different heights. 8.Apparatus according to claim 6, wherein said shaped body (10) isoperated so as to be set in rotation at a predefined speed.
 9. Apparatusaccording to claim 6, wherein the rotation axis of said disc is inclinedwith respect to the meridian plane of said tire.
 10. Apparatus accordingto claim 5, wherein said surface (4) is a rotating drum.
 11. Apparatusaccording to claim 5, wherein said arm (5) is movable to be translatedso as to vary the load applied on said tire (2).
 12. Apparatus accordingto claim 5, wherein said shaped body (10) penetrates said tire sidewallin proximity of the footprint area thereof with said surface (4).